TL;DR: Godwin's Law, but with fantasy villains
SPOILER: Just don't read this if you don't want to be spoiled. Nearly every big bad in Mainline is listed here.

I have found that, while Wesnothian campaign and scenario guides are wonderful for most purposes (especially the one by ESR), they (unless I have embarrassingly overlooked something on this forum) are deafeningly silent on a matter of vital importance: cussing.

I am so mortified by this that I have decided to swallow my pride and write a family-friendly family-friendly guide to cussing (seriously guys, this is weird).

Let me begin by noting that cussing is a vital part of our unenlightened non-discourse IRL. We cuss when we are in pain. We cuss at people, institutions, or decisions we don't like. We cuss to express extreme emotions. We don't actually cuss as much as this paragraph seems to indicate, but we do it often enough that there exist actual linguists making a living off of the study of our potty-mouths.

Academics note that cuss words are often built upon a taboo. Languages all over the world have cuss words based on various taboo bodily phenomena (for example, one of Italy's governing parties uses such a word as a political slogan). But these taboos are too obvious, and the phenomena behind them too vulgar, for them to be of much use to us. The best one can hope for while being family-friendly is to refer to some disease which is customarily spread in an age-appropriate manner, such as the yellow fever, smallpox, malaria, or the plague.

But there exists another taboo to base a cuss on: a stigmatized group or person. This is especially useful because such words are often extremely characterizing IRL (so much that you can't go two weeks without some famous person being ostracized for using one of the particularly nasty and sadistic ones). So bigots can use made-up racial slurs: dirtgrubber is ESR's go-to for his dwarf supremacist villains in THoT. Better yet, arrogant or extremely angry characters can use words like weakling, imbecile, idiot, stripling boy, silly girl, jackal, or maggot to insult someone's body, intellect, age, lack of a Y chromosome, trustworthiness, or stature.

The best insults, however, are those which use a stigmatized group which actually earned its stigma. For example, the astonishingly evil actions of a certain Charlie Chaplin-lookalike turned said doppelgänger into a universally-recognized symbol of evil. We draw his mustache on images of celebrities we dislike, associate his favorite symbol with things we dislike (especially Comcast), and use the name of his political party to insult anybody we dislike. The tendency of our unrelenting cultural hate regarding this hateful man to invade our discourse even has a name: Godwin's Law.

Therefore, I believe that the best cuss words (even better than cuss words from real life) will arise from referring to some or another infamous figure in Wesnothian history in conjunction with an age-appropriate body part, personal possession, or personality trait.

Here's a list containing cusses derived from almost every Wesnoth Mainline Campaign big bad ever: (accompanied by very poor jokes)

Ravanal's Ashes (this is why null stones have been banned in California)
Jevyan's Spine (doctors hate him: this one trick fixes your back pain permanently (accepting lichdom and putting a jar of olives between your ribs))
M'Brin's Vengeance (apparently elven health insurance doesn't pay for therapy either)
Asheviere's Evil Eye (guys, I think its watching me through the fourth wall)
Yechnagoth's Tendrils (apparently gangrene is even more disgusting in elder gods than in humans)
Malifor's Scalp (he was saving that 10,000 gold for the copay on his next miracle hair treatment)
Brurbar's Beard (what's left of it after Kalenz covered it in chewing gum and tooth paste overnight)
Landar's Fury (remember kids, if you try liches' eyeballs even once, you will never be the same)
Iliah-Malal's Brain (as it turns out, destroying the border between the underworld and the living world can cause neurological issues)
Karrag's Pupils (even liches get glaucoma)
Rualsha's Rabid Rage (he should consider joining M'Brin's anger management support group)
Rakshas' Guts (he doesn't have any)